Mechanical wave filter



Sept. 17, 1935.

INVE/VTOR H.W.AUGUSTADT BY A T TORNE Y p 1935. H. w. AUGUSTADTMECHANICAL WAVE FILTER Filed Feb. 28, 1933 2 Sheets-Shee INVL'NTOR H. W.AUGUS TA 0 T A TTORNE Y Patented Sept. 17, 1935 UNITED STATES PATENTOFFICE MECHANICAL WAVE FILTER Application February 2s, 1933, Serial N0.658,929

11 Claims.

This invention re1ates 120 frequency selective systems and moreparticularly to mechanical.

wave filters, that is, mechanical systems for selectively transmittingvibratory wave motion.

An object of the invention is the selective transmission of a singlefrequency er a narrow band cf frequencies.

Another object is to improve the attenuation characteristics ofmechanical wave filters.

Anot her object is 150 se1ect mechanically two or more narrow frequencybands and at the same time to attenuate strongly one or moreintermediate frequencies.

A further object is to simplify the coupling between a mechanical fi1terand the associated electrical circuit.

Still another object of the invention is so reduce the weight and cost0f mechanical wave filters used in connection with electrical circuits.

A feature of the invention is a wave filter adapted to be inserted in anelectrical circuit, but the filtering action o:f which is dependent uponmechanical means.

Another feature is a filter in which electrica1 energy is converted intomechanical and the mechanical energy is reconverted int0 electrical bymeans of a single magnetic field structure.

When a mechanical filter is used as a component part of an electricaltransmission system it is, of course, necessary in some manner to couplethe mechanical 130 the electrical circuit. Heretofore, two separatefield structures have been employed Tor this purpose, one for converting the electrical energy into mechanical energy and the second forreconverting the mechanical back into electrical energy. In accordancewith the present invention this energy conversion is efiected with asingle magnetic field, in which both the input and the output coils ofthe filcer are constrained 130 vibrate. In this way the couplingarrangement betvveen the mechanical filter and its associated electricalcircuit is simplified with a consequent reduction in the required weightand cost of the filter, due to the complete elimination 01 one fieldstructure. If a sing1e frequency er narrow band 0f frequencies is to betransmitted the mechanical system may be given a single resonantfrequency. If two o1 more frequencies are to be passed the mechanicalsystem may be provided With the requisite number 0f resonances, and theintermediate antiresonant frequencies may be so placed as to attenuatestrongly undesired frequencies. An applicatiori of the invention hasbeen found in Connection With navigation aids f01 aircraft where,

.a gap I5 and may be secured in place o n the for example, it isrequired to separate frequencies 0f and 86 cycles per second,respectively. F01 a description of an aircraft radio beacon system towhich the invention is applicable reference is made an article by J.I-I. Dellinger and I-I. 5 Pratt, in the Pr0ceedings of the Institute ofRadio Engineers, V01. 16, page 890, July, 1928. In accordance with theinvention a pair of mechanical filters may be provided. to accomplishth1's frequency separation. One of the filters is de- 10 signed 130transmit a fre quency o-f 86 cyeles while having a peak 01 attenuationab 65 cyc1es; the

other fi1ter is designed to transmit a frequency of 65 cyc1es, With anattenuation peak at 86 cycles. In this way a 1arge amount ofdiscrimination between the two frequencies may be provided. The filterof the invention is particularly adapted for use on aircraft because ofits simple, compact construction, lightness 0f weight anal reliabilityof operation.

The invention will be more clearly understood from the followingdetailed description and the accompanying drawings, of which:

Fig. 1 is aperspective view, partially cut away, of a mechanical filterembodying the invention;

Fig. 2 illustrates a modified form of the filter shown in Fig. 1;

Fig. 3 is a detailed view of an element used in the filter of Fig. 2;

Fig. 4 represents diagrammatically the chanical system of Fig. 2; and

Figs. 5 and 6 are attenuation characteristics obtainable With thefilters shown, respectively, in Figs. 1 and 2.

Referring especially to Fig. l, one form of the 35 mechanical fi1ter ofthe invention comprises an input 0011 l i and an output coil I2 wound ona light form er drum 13 which is mounted 011 a metal band M andconstrained 130 vibrate in the airgap 15 of the permanent magnet I6. Thecentral por- 49 tion H of +he band M is enlarged 0-1 reinforced in orderto add rigidity to the coil form I3. Duralumin has been found be asatisfactory material from which to make the band M as it is light inweight and has sma1l internal dissipation 45 losses. One end 0f the bandI4 is clan1ped the bracket; i 8 by means of the block 99 and ohe screws20. The ether end of the band H3 fits into a s1ot in the square block 2land is he1d by the screvv 22. The outer end of the block Ei 50terminates in a rounded portion which is threaded. 110 receive the thumbnut 23. The block 2l has a sliding fit in a square hole in the bracket24. The coil form I3 is carefu1ly centered in the ing band measured a1;the same point. In equation form 1 21r /mc where f1 is the frequency incycles per second, m is the mass in grams and c is the compliance incentimeters per dyne. For a given mass m lt is apparent that thefrequency of resonance may readily be adjusted by increasing ordecreasing the tension of the band I4, which may be accomplished bytightening or loosening the thurnb nut 23. The nat-u1al frequency of themechanical system is in this way set at the frequency to be transmittedby the filter. In Operation, the alternating electromotive force isimpressed upon the terminals 26 of the input coil II and if frequency f1is present, the mechanical system is sei; into vibration by the actionof the alternating current flowung in the coi1 winding II. The vibratorymotion of the output coi1 I2 in the field of the magnet I 6 induces analternating electromotive force of corresponding frequency in the coilWinding I2 which is effective at the output terminals 2I. The lead wiresfrom the coil winding I2 I'0 the terminals 21 may be placed inside thedrum I3 and brought out through the hole 28 as shown in Fig. 1. Due tothe unavoidable mutual inductance between the two coils II and I2 thereWill be a small direct transfer of energy of frequencies other than 71,if present, but due to: the small number of turns required. for thesecoils they act very inefficiently as a transformer, and therefore thisby-passing of undesired frequencies may safely be ignored. Thus, thefilter will pass freely currents of frequency f1 but Will greatlyattenuate all frequencies slightly rernoved to one side or the other ofthis frequency. A possible attenuation characteristic of the filtershown in Fig. 1 is given by the curve of Fig. 5, in which frequency incycles per second as abscissae is plotted against the attenuation indecibels as ordinates, with the frequency of resonance denoted by f1.Attenuations as high as 45 decibels are obtainable with the filter shownin Fig. 1. If coi1s II and I2 each have the Same number of turns of wirethere Will be no impedance transformation within the filter. If desired,however, an impedance transformation may be introduced by a properchoice of the turns ratio between the two coil windings.

A modification of the invention is shown in Fig. 2, in which a tunedreed 28, clamped at its base to an extension of the brackel; 24, iscoupled to the band I4 by the spring 30. The reed 29, as shown in detailin the perspective view of Fig. 3, has flanges 3I along the sides, Withalle exception o-f the narrowed portion 32, so that bending will beconfined to this latter region. The motion of the reed is to and fro, inthe same direction as that of the band I4. The end portion 33 of thereed 29 is clamped between the two blocks 34 and 35, as shown in Fig. 2.The block 34 is secured. to the bracket 24 by the bolts 36, the reed 29fitting into a 1*ecessed portion of the block 34 and being clamped inplace by means of the block 35 and the two screws 3'I. One end of thespring 30 is clarnped to the band I4 at the center by means of the block38, which is held by the two screws 39. The other end of the spring 30is clamped to= the end of the reed 29, the end of Ehe spring being heldby the block 40 and the two screws 4 I.

The mechanical system shown in Fig. 2 is represented diagrammatically byFig. 4, in Which m1 is the effective 1n-ass of the band I4 andassociated coils at the point of drive, c1 is the compliance of the bandI4, mz is the effectiv-e mass of the reed 28 ab its point of drive 02 ische compliance of the spring 30 and ca is the compliance of the reed 29.The arrow F indicates the point of application 0f tne driving force. Insuch a system there Will, in general, be two frequencies of resonance or10W impedance f1 and f3, and an intermediate frequency of anti-resonanceor high impedance f2. An attenuation characteristic obtainable with thefilter of Fig. 2 is shown in Fig. 6, where lt is seen that the resonantfrequencies fi and f3 corresponcl with pass-bands, and the anti-resonantfrequency 72 coincides with a peak of attenuation. The criticalfrequencies f1, f2 and ja may be placed as desired provlcled ehe threecompliances 01, 02 and C3 are proportioned according to the followingequations:

II; Will be noced that the masses m1 and mz are not uniquely determinedin the above three equations. The values of m1 and mz may be assignedarbitrarily, o1 they may be so chosen thab the filter Will present adesired impedance at some specified frequency. If a sharp resonancecurve is desired, that is, a steeply rising attenuation characteristic,then the ratio of m1 o0 c1 should be made large; if, on the other band,greater maximum losses ar.e desired, and a wider transmission band ispermissible, then the ratio of m1 to 01 should be small.

The filter illustrated in Fig. 2 is particularly adapted for use intransmitting one or owo single frequencies, or narrow bands, whileeffectively excluoling a third frequency. For example, two frequencies]A and fB, both p1esent on a transmission line, may be separated bymeans of a pair of such filters ccnnected in parallel across the line.In the filter which is to pass the frequency fA the first resonance 11may be placed ab the frequency fA, and the peak of attenuation ab f2 maybe chosen to coincide with the frequency f. In the other filter, whichis to transmit the frequency JB, the resonance f; is sei; at I:hefrequency fB while the anti-resonancc f2 is placed at the frequency A.The first filter Will then pass freely the frequency fA but will excludeeffecively uhe frequency fB, and tne second filter Will pass fB whilestopping fA.

II; is readily apparent that a filter having any desired number ofpass-bands, With the attendant intermediate peaks 01 attenuation, may be0btained simply by providing the requisite number of additional reedsand connecting springs. In general, each addition of a reed and springadds another transmission band and another attenuation peak.

What is claimed is:

1. In a wave filter, means for converting electrical energy intomechanical energy and for reconverting the mechanical energy intoelectrical energy, comprising a magnetic circuit having an air gap, anon-dissipacive vibratory member resonant to the frequency of the signalenergy to be passed, an input coil and an output coi1 carried by saidvibratory member and constrained to vibrate in said air gap, whereby anarrow band cf frequencies corresponding 130 the resonant frequency ofsaid vibratory member is free1y transmitted while frequencies lyingoutside of said band are highly attenuated.

2. Apparatus for the selective transmission of vibratory energycomprising a vibratory member sharply tuned to be resonant to the Samefrequency as that of a periodic signal current; to be transmitted, amagnetic circuit providing an air gap, an input coil constrained tovibrate in said air gap for actuating said member under the control ofsaid signa1 current, and an output coil, also ionstrained to vibrate insaid air gap unde1 the control cf said member for producing a vibratorycurrent of the same frequency of said signal current, whereby thefrequency corresponding to the resonant frequency of said vibratorymember is freely transmitted while fre quencies lying to either side ofsaid frequency are highly attenuated by said apparatus.

3. A wave fi1ter comprising a vibratory member having a naturalfrequency cf vibration, means for producing a magnetic field, an inputcoil and an output co-il, said coils being mounted on said vibratorymembr and said coi1s being constrained vibrate in said magnetic field,whereby a current having a frequency corresponding to the naturalfrequency of said vibratory member, when impressed upon said input coil,will indu ze in said output coil a current of corresponding frequency,whereas currents of other frequexicies, when impressed upon said inputcoi1, Will be effectively blocked by said wave filter.

4. A wave filter comprising a plurality of vibratory members coupled bymeans of springs, said connected system of vibratory members having aplurality o:f natural frequeneies of Vibration, means for producing amagnetic field, and an input coi1 and an output 0011 mounted 011 one ofsaid vibratory members; said coils being Constraihed vibrate in saidmagnetic field, Whereby the said filter Will freely transmit currentscorresponding in frequency 130 the natura] frequencies of said system cfvibratory members but Will greatly attenuate currents having frequenciesslightly removed from said. natural frequencies.

5. A frequency se1ector comprising a nun-dissipative vibratory memberresonant tothe same frequency as that of a periodic signal current,means for producing a magnetic field, an input 0011 and an output coil,said coils being associated with said vibratory member and movingtherewith, and said coils being constrained to vibrate in said magneticfield, whereby the said sigma] current is free1y transmitted from saidinput to vibrate in said magnetic field, whereby said fi1ter is adaptedto pass freely said. signal eurrent from said input coil to said outputcoil while substantially stopping the current of said undesired.frequency.

'7. A wave filter comprising a vibratory member sharply resonant 130 thesame frequency as that of a periodic signal current to be transmitted,means for adjusting the resonant frequency of said member, means forproducing a magnetic field, an input coil and an output coil, said coilsbeing 1ahysically associated with said. member and being adapted 130vibrate ab the same frequency as said member, and. said coi1s beingconstrained to vibrate in said magnetic field, whereby said. signalcurrent is passed by said filter With comparatively small 1oss buteurrents of frequencies 1ying slightly to either side of the frequencyof said signa1 current are greatly attenuated by said filter.

8. A wave filter comprising a magnetic circuit having an air gap, a bandthe resonant frequency of which may be sharply tuned, a 0011 formmounted on said. band and. extending into said air gap, an input coiland an output coil both carried by said coil form, whereby a currenthaving a frequency corresponding to the resonant frequency of said bandwill be freely transmitted. from said input coil to said output coi1,whereas currents of other frequencies Will be effectively blocked bysaid filter.

9. A wave filter comprising a, magnetic circuit providing an air gap, aband having an adjustable tension, a coil form mounted on said band andextending into said air gap, two windings carried by said coil form, anda tuned reed.mechanically coupled to said band by means of a spring.

10. A wave filter comprising means for producing a magnetic fie1d avibratory member having a natural frequency of vibration, an input andan output coil both mounted on said member and constrained to vibrate insaid magnetic field, and a second vibratory member mechanically coupledto said first member by means 0f a spring.

11. A wave filter comprising means for producing a magnetic field, avibratory member having a natural frequency of vibration, means foradjusting the frequency of Vibration of said member, an input coi1 andan output coil both mounted on said member and constrained to vibrate insaid magnetic field, and a second Vibratory member mechanically coupledto said first member by means of a spring.

HERBERT W. AUGUS'I'ADT.

